On Changes in the Sea and Their Relation to Organisms. 



227 



content in the afternoon than in the morning. Since oxygen is un- 

 doubtedly produced by these plants in the sunshine, the failure to 

 observe an accumulation during the day probably Hes in the fact that 

 the temperature of the water rises about 1°, thus reducing the absorp- 

 tion coefficient for O2 by 0.08 c.c. per liter and causing a passage of 

 O2 into the air. In contrast to this behavior of deep water, the O2 con- 

 centration of the shallow water of Tortugas varied from 3 to 4.5 at 

 about dawn to 4.5 to 7.0 at 3 p. m., the average maximum being shown 

 in figure 4 opposite the station letters. 



DISCUSSION. 



Determinations [of the pH from certain North Atlantic stations, 

 table 8, made by Palitzsch (1912 b) compare favorably with those I 

 have made in the Gulf Stream and in the deep water near Tortugas. 



Table 8. 



There is no increased amount of phytoplankton at Tortugas, but 

 attached seaweed and symbiotic algse and diatoms at the bottom cause 

 the great diurnal change in O2 concentration. On coral reefs the sym- 

 biotic algse of corals and actinians are very effective, and in lagoons 

 or other water which is not too agitated the symbiotic algse of two 

 species of the bottom medusa Cassiopea are a significant factor. One 

 Cassiopea xamachana (11 cm. in diameter, weighing about 117 grams) 

 in the sunlight gave out 1.9 c.c. O2 per hour, whereas in the dark it 



